1. Field of the Disclosure
The present invention relates generally to fluid flow and control, and more particularly, to a pinch valve having a reduced-friction guide mechanism.
2. Description of Related Art
A fluid flow control system generally consists of three components: a flow sensor, a control valve and a controller such as a proportional-integral-derivative (PID) controller. A typical fluid flow control system functions by changing the amount of opening of the control valve until the flow sensed by the flow sensor matches the desired, or set point flow. Often, the control valve must be adjusted by very small amounts to achieve the desired closeness of control.
Many industries such as semiconductor, pharmaceutical, and bio-technology experience fluid control problems due to the typically low flow rates, the use of abrasive chemical fluids, the use of corrosive chemical fluids, and the need for contaminant free, accurate, compact, and real-time fluid control and delivery systems.
For example, Chemical-Mechanical Planarization (CMP) is a critical process in the semiconductor industry that involves a process to flatten the wafer surface of a semiconductor by applying an ultra-pure fluid containing suspended solid particles and a reactive agent between the wafer surface and a polishing pad. In most applications, the polishing pad rotates at a controlled speed against the semiconductor to flatten the surface. Over-polishing the wafer can result in altering or removing critical wafer structures. Conversely, under-polishing of the wafer can result in unacceptable wafers. The polishing rate of the wafer is highly dependent upon the delivery rate of the fluid and the total amount of fluid delivered during a polishing operation.
Another process used in the semiconductor industry requiring accurate control of fluid flows and a contaminant free environment is the photolithography process. As is known in the art, photolithography is a process that applies a light sensitive polymer, known as resist, or photo resist, to the wafer surface. A photomask containing a pattern of the structures to be fabricated on the wafer surface is placed between the resist covered wafer and a light source. The light reacts with the resist by either weakening or strengthening the resist polymer. After the resist is exposed to light the wafer is developed with the application of fluid chemicals that remove the weakened resist. Accurate and repeatable resist delivery is essential to properly transfer the pattern. The resist must be contamination free as any dirt on the surface will cause a defect in the final pattern.
A modification of this process applies a host of new liquids to the wafer surface to create films that will become an integral part of the final semiconductor. The primary function of these films is to act as an insulator between electrical conducting wires. A variety of spin-on materials are being evaluated with a wide variety of chemical compositions and physical properties. The key difference between the lithography process and the spin-on deposition is that any defect in the film (such as a void, bubble or particle) is now permanently embedded in the structure of the semiconductor and could result in non-functioning devices and a financial loss for the semiconductor producer.
Both of these processes take place in a tool called a track. The purpose of the track is to apply a precise volume of fluid to the surface of a stationary or slowly spinning wafer. Additional chemical processing steps may be used to convert the liquid to the proper structure. After the liquid application, the wafer rotation speed is rapidly increased and the liquid on the wafer surface is spun off the edge. A very thin, consistent thickness of liquid remains from the center of the wafer to the edge. Some of the variables that affect liquid thickness include the resist or dielectric viscosity, solvent concentration in the resist or dielectric, the amount of resist/dielectric dispensed, speed of dispense, etc.
The track will also provide additional processing steps after liquid application that changes the liquid to a polymer using a bake process that also removes any solvent in the film. The track also controls the environment around the wafer to prevent changes in humidity or temperature and chemical contaminants from affecting the performance of the film. Track system performance is determined by the accuracy and repeatability of liquid delivered to the wafer surface in addition to minimizing defects in the film caused by voids, bubbles and particles.
The fluid control element is thus a critical component of such systems to insure proper delivery of the process fluids. A pinch valve may be used for the fluid control valve in such systems to provide an efficient, compact and high purity fluid control device. In particular, a solenoid-actuated pinch valve provides a cost-effective means for providing fine fluid control. Such valves typically have a guide mechanism that incorporates parts that slide against each other. With any sliding mechanism there is a finite amount of friction. Even if the sliding parts are made up of low friction materials, there will still be some friction. This friction can result in a stick-slip motion of the pinch valve, impacting the precision of flow control.
The present application addresses shortcomings associated with the prior art.